The invention relates to marine power plant systems including separate turbines arranged for ahead or astern operation and wherein the ahead turbine operates on a reheat vapor cycle.
Problems arise with the application of the reheat cycle to marine power plants due to the fact that separate turbines are required to operate a vessel in the ahead and in the astern directions. The need for separate turbines to operate the vessel in each direction creates much difficulty and in most cases even prevents the use of a reheater due to the fact that when steam flows to the astern turbine vapor supplied to the ahead turbine must be discontinued and this in turn means that vapor flow to the reheater is also discontinued thereby starving the reheater tubes of cooling fluid. With no cooling fluid flowing through the reheater at these times the tubes are subject to deterioration by burnout.
It has been the practice in marine power plant installations operating on a reheat cycle to protect the heat exchange tubes that comprise the reheater tube bundle against overheating and burnout during periods of astern maneuvering in one of two principal ways. First, the reheater tubes have been cooled by passing an amount of superheated vapor through them during these periods wherein the heat picked-up in the tubes is transferred via the vapor to an external heat exchanger or to a spray desuperheater. This manner of reheater tube protection has gained little acceptance due to the fact that a complex, expensive valve and piping arrangement is required to reroute the superheated vapor flow during periods of non-reheat operation.
Alternatively, protection schemes have involved isolation of the reheater tube bundle wherein the same is physically removed from the main section of the vapor generator. In these arrangements the reheater is housed in a separate chamber, normally formed by division walls in the furnace, and heated during periods of operation by independent, supplemental burners. Such schemes are not totally acceptable due in part to the fact that, since only a portion of the total combustion gas is caused to flow to the reheater, inordinately high gas temperatures are normally employed including a significant amount of radiant heat input because of the close proximity of the burners to the tube bundle. This high heat input requires the tube bundle to be designed with an undesirably high pressure drop which has an adverse affect on cycle efficiency. Or, if pressure drops are kept low, then poor vapor distribution is a potential hazard giving rise to the danger of tube failure.
It is to the improvement of such marine power plant systems, therefore, that the present invention is directed.